1. Field of the Invention
The present invention relates to a substrate for a magnetic disk, which is useful for a hard disk or a flexible disk, and a process for its production. More particularly, it relates to a substrate for a magnetic disk suitable for a continuous thin film of metal type magnetic recording medium.
2. Discussion of Background
A magnetic recording medium using a thin magnetic film of metal such as Co--Cr alloy or Co--Ni alloy is expected to be useful as a high density recording medium in view of its high saturation magnetic density and surface smoothness, and it is studied for a wide range of practical applications in various regions. In particular, Co--Cr having perpendicular magnetic recording properties has been developed to have a linear recording density as high as 200 KBPI. Further, Co--Ni has been reported to have a linear recording density of 70 KBPI when used as a thin film horizontal magnetic recording medium having a thickness of about 500 .ANG..
Both of the above-mentioned magnetic recording media are far superior to the most commonly employed .gamma.-Fe.sub.2 O.sub.3 coating type medium, the linear recording density of which is from 15 to 20 KBPI.
However, a continuous thin film medium having such excellent magnetic recording properties has not yet been practically widely employed. One of the factors preventing the practical application is a problem concerning the sliding properties of the magnetic head and the medium.
Namely, a continuous thin film medium made of metal has a smooth surface, and it is likely to lead to magnetic head crush due to sticking of the magnetic head and the medium in the case of a hard disk and it is likely to lead to magnetic head crush due to an increase of the friction coefficient during the sliding operation because of the difficulty in maintaining the lubricant in the case of a flexible disk.
To solve these problems, it has been attempted to apply mechanical texture treatment i.e. surface treatment of imparting linear scratch marks to the substrate surface by means of e.g. sand paper. FIG. 13 is a differential interference microscopic photograph (400 magnifications) showing the surface texture of a substrate after the mechanical texture treatment, and FIG. 14 is an enlarged perspective view of an area of 248 .mu.m.times.248 .mu.m of the same surface texture by means of a laser interference-type roughness meter.
This mechanical texture treatment is effective to some extent. However, in the case of a medium where high density recording is required, it is likely to lead to an increase of bit errors depending upon the density or the depth of the texture (linear scratch marks), whereby it is difficult to control the bit errors at a mass production level. Namely, if the track width of the magnetic recording is as narrow as from 10 to 20 .mu.m, the modulation (the change in the reproduction output when the recording current is varied) tends to exceed 10% unless the width of the texture is made to be at a level of 1/10 of the track width. Further, if the depth of grooves of the texture exceeds 200 .ANG., the head output tends to decrease by 10% or more, whereby the modulation tends to increase. Even if a texture satisfying these requirements can be obtained, if the number of linear marks per unit area is small, there will be a problem of sticking since the sliding area of the magnetic head is as large as 0.5 mm.times.4 mm. Further, because of irregularities in the roughness such as burrs formed by the mechanical texture treatment, the distance between the head and the magnetic recording medium can not adequately be shortened. Accordingly, it is difficult to increase the recording density due to the spacing loss. Especially in the case of a hard disk, such irregularities in the roughness produce dust during CSS operation, and such dust adheres to the head and the medium, thus causing head crush.